Changes of cyclic AMP levels and phosphodiesterase activities in the rat ovary

Cell-specific expression and immunolocalization of nitric oxide synthase isoforms and the related nitric oxide/cyclic GMP signaling pathway in the ovaries of neonatal and immature rats

OBJECTIVE

The present study is designed to investigate the cellular expressions and immunolocalizations of three different nitric oxide synthase (NOS) isoforms and the related nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling pathway in the ovaries of neonatal and immature rats.

METHODS

The ovaries were obtained from ICR (Institute for Cancer Research) female Sprague-Dawley rats at postnatal days 1, 5, 7, 10, and 19. Then we carried out the histologic examination, immunohistochemistry, measurement of NOS activity, and modifications within the NO/cGMP pathway.

RESULTS

During postnatal days 1, 5, 7, 10, and 19, all three isoforms of NOS were mainly localized to the oocytes and expressed as a gradual increase in granulosa cells and theca cells within the growing follicle. The ovarian total NOS activities and NO levels were increased at postnatal days 7 and 10 compared with other days.

CONCLUSIONS

Our findings suggest that the locally produced NO and the NO/NOS signaling systems are involved in the follicular development to puberty.

Activation of PI3K/mTOR signaling pathway contributes to induction of vascular endothelial growth factor by hCG in bovine developing luteal cells

We recently reported that HIF-1α plays a critical role in the regulation of vascular endothelial growth factor (VEGF) expression in the developing letual cells (LCs) and VEGF-dependent angiogenesis is essential for normal luteal development. Although it is believed that hypoxia is the primary inducer of VEGF, recent reports have also shown that human chorionic gonadotrophin (hCG) up-regulates VEGF expression in developing corpus luteum (CL). Therefore the present study was designed to test the induced effects of hCG on the expression of VEGF and HIF-1α in LCs under normoxic and hypoxic conditions. In addition, we also investigated whether the signaling pathways such as phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK) are involved in hCG-induced VEGF in LCs. A significant increase of VEGF mRNA was found in LCs treated with hCG, which was consistent with the changes of HIF-1α protein, even under hypoxic conditions. However, there was no obvious changes of HIF-1α mRNA in hCG-treated LCs between normoxic and hypoxic conditions, indicating hCG induces VEGF expression by increasing transcription of HIF-1α, while hypoxia mainly increases HIF-1α protein stability. When LCs were pretreated with inhibitors, we found that the PI3K/mTOR signaling pathway is required for HIF-1α and VEGF expression induced by hCG, while the MAPK pathway is not required. Together, these results suggest that activation of IP3K/mTOR signaling pathway contributes to the induction of VEGF and HIF-1α in hCG-treated LCs. To our knowledge this will provide a new insight into the important mechanism of hCG/LH-induced VEGF-dependent angiogenesis in the bovine ovary.

Effects of puberty and gonadotropins on the molecular events controlling meiotic resumption of mouse oocytes

Although studies suggest that the low competence of oocytes from prepubertal animals is due to their insufficient cytoplasmic maturation and that FSH improves oocyte maturation possibly by retarding meiotic progression and allowing more time for cytoplasmic maturation, the mechanisms by which puberty and gonadotropins regulate meiotic progression require additional detailed studies. For the first time, we observed that while meiotic progression was significantly slower, the maturation-promoting factor (MPF) activity of oocytes was significantly higher in prepubertal than in adult mice. To resolve this contradiction, we specified the molecules regulating the MPF activity and their localization during oocyte maturation in prepubertal and adult mice primed with or without gonadotropins. Our tests using corresponding enzyme regulators suggested that while activities of protein kinase A were unaffected, the activity of adenylate cyclase (ADCY) and phosphodiesterase increased while cell division cycle 2 homolog A (CDC2A) decreased significantly after puberty. While most of the adult oocytes had CDC2A protein concentrated in the germinal vesicle (GV) region, the majority of prepubertal oocytes showed no nuclear concentration of CDC2A. Maximally priming mice with equine chorionic gonadotropin brought the above parameters of prepubertal oocytes close to those in adult oocytes. Together, the results suggest that puberty and gonadotropin control oocyte meiotic progression mainly by regulating the ADCY activity and the concentration of the activated MPF toward the GV region.